Edge or line jaggedness is a common problem associated with the printing of low resolution image data. Several methods of processing low resolution image data have been devised in attempts to overcome the line jaggedness problem. For example, in the case of images created from low resolution binary image data, template matching methods of the type described in U.S. Pat. No. 5,005,139 or outline extraction methods of the type described in the article entitled "Image Makers" by Henry Bortman, MacUser, November 1991, pp. 98-103 have been used to reduce line jaggedness. Both of the above-referenced methods use a low resolution bitmap as an input file and both methods perform digital processing operations on the low resolution bitmap to produce an enhanced image output file. The enhanced output file is supplied to the printhead of a printer to produce a hardcopy of the image.
There are disadvantages, however, to both of the template matching and outline extraction methods of edge enhancement discussed above. In the template matching method, the number of templates that can be employed must be limited in order to maintain acceptable processing speeds. If a pattern within the bitmap data contained in the input file does not happen to match one of the available templates, an edge resolution enhancement operation cannot be performed, regardless of the quality of the image that will be produced from the input data. In the outline extraction method, variable dot sizes are used to "fill in" and match a character's outline. The fill in process, however, can be difficult when small fonts are employed. The outline extraction method also tends to broaden thin graph lines that are spaced close together, which results in the merger of multiple lines and a perceivable reduction in the resolution of the printed image.
Another drawback to the above-described processes is that they tend to have a negative impact on gray-scale halftone data. Thus, the processes are not particularly well suited for processing mixed image data files containing both binary image data and gray scale halftone image data. Mixed data files, however, are becoming more and more common. Many imaging systems, for example, now combine text data with gray-scale halftoned image data and binary graphic data in a single image.
In view of the above, it is an object of the invention to provide a system for providing edge enhancement of mixed image files containing both binary image data and gray-scale image data without suffering the resolution degradation associated with the outline extraction method and the drawbacks of the template matching method outlined above.